Electrochemical cells employing liquid cathodes and high energy density anodes find use in many devices requiring high power and long service life. In some cell constructions, a cathode collector is disposed next to a cell housing. In contact with the cathode collector is a separator; and an anode is in contact with the other side of the separator. As the cell discharges, the anode is consumed. This can create a problem as space results between the anode and separator or the cathode collector and separator.
One solution to the problem is disclosed in U.S. Pat. No. 4.032,696. That patent discloses liquid cathode cells having at least two discrete anode bodies. A spring tab has biasing means and is in contact with a current collector on the anode bodies. As the cell discharges, the spring tab biases the anode bodies outwardly against the separator which in turn contacts the cathode collector.
Although this assembly was an improvement over previous assemblies, the cells frequently exhibited a sudden voltage drop. This problem resulted from loss of electrical contact between the spring tab and anode. The anode current collector typically employed is a metal screen or grid having relatively small openings of generally less than about 0.08 inches measured lengthwise to establish uniform pliable contact along the complete surface of the anode. The spring tab would remain in contact with the anode current collector, but the voltage drop would still occur due to the formation of an insulative passivation layer on the anode current collector, which is formed on the exposed portion of the anode current collector upon contact with the cathode-electrolyte. This layer is desirable, because it prevents the self discharge of the anode and liquid cathode. However, as the normal discharge reaction proceeds and the anode is consumed, the spring tab can shift along the anode current collector and can become in contact with a passivated portion of the anode current collector. Thereby, electrical contact will be lost.
The loss of electrical contact phenomenon can occur in greater than 35% of cells. To measure this phenomenon, cells are discharged to a cutoff voltage, and the time to cutoff is measured. In a given sample of cells, a large standard deviation indicates a high occurrence of loss of contact. For example, a sample of 25 cells which are discharged at 250 ohms to a cutoff voltage of 2.7 volts has a mean time of 4377 minutes, a maximum time of 4947 minutes and a minimum time of 3310 minutes. The standard deviation is 499 minutes, which indicates a variation in the discharge time of about .+-.34% of the mean discharge time. That is, the discharge time can be expected to vary by 68%. In a sample of 25 cells which are discharged at 250 ohms to a cutoff voltage of 1.8 volts, the mean time is 4874 minutes, the maximum time is 5543 minutes and the minimum time is 3619 minutes. The standard deviation is 525 minutes, which indicates a variation in the discharge time of about .+-.32% of the mean discharge time.
The problem of spring tab shift to passivated portions of the anode current collector cannot be solved by removal of the anode current collector. In addition to providing for the uniform collection of current for the anode, the anode current collector assists in the insertion of the spring tab, without the anode current collector, the spring tab will tear and gall the anode metal when the spring tab is inserted. Also, during deep discharge, continuity of contact would be lost.
In view of such disadvantages, it would be desirable to have electrochemical cells which employ a biasing tab contact and a consumable anode in which the tab would remain in electrical contact with the anode during discharge of the cell.